JavaFX has different selectors and some of web css doesnt work with it. Problem is the Javafx css engine doesn’t support structural pseudo-classes, so javafx just ignore it. You should set CellFactory where you by index decide, here solution i guess:

   myComboBox.setCellFactory(lv -> new ListCell<String>() {// Here you have one method and there set your behaviour np check + by f/s index apply styles})